The present invention is based on algorithms previously proposed in the literature. The invention concerns a number of algorithm modifications, which overcome some of the limitations of other systems used for feedback reduction in hearing aids.
The invention relates to a feedback cancellation algorithm which does not need an artificial noise signal in order to estimate the feedback transfer function. The input signal received from the environment, or the feedback oscillation signal, is used to drive the estimation process. In this fashion, the hearing aid user does not listen to an added noise signal, and a higher sound quality is possible. However, it is well known that such ‘no-noise’ algorithms can have audible side effects under certain circumstances, especially when environmental signals with long autocorrelation functions are present at the microphone.
The autocorrelation function for a signal describes the average correlation between two signal values, which are separated by a time difference “Lag”. In loose terms, the autocorrelation function describes how “predictable” a signal value is, given the other samples in the signal. Some signals, for example periodic signals, are highly predictable and, correspondingly, the autocorrelation function does not vanish even for large values of Lag. Other signals, such as white noise, are very little predictable, and their autocorrelation function quickly vanish for increasing values of Lag. For signals with a long autocorrelation function, a future sample value can be predicted with a high degree of confidence, given the past samples. In other words, new samples of the signal do not provide much new information. Careful analysis of feedback cancellation systems reveal that signals with long autocorrelation may drive the adaptive system to produce poor estimates of the feedback path.
It is an objective of the present invention to provide a method and a hearing aid having means for feedback cancellation which improves the result of the feedback canceling by being more stable and thereby gives an improved user comfort.